1. Field of the Invention
The present invention relates to the measurement of electrical current through a conductor, and more specifically to the measurement of current through an interconnection line on a semiconductor device.
2. Background Information
In the semiconductor industry, it is important to characterize the internal current flow in a semiconductor device such as, for example, a microprocessor. Often, computer simulations are performed to determine the current flow under various operating conditions. With the knowledge of the current flow, and therefore the power distribution, various performance and reliability concerns can be addressed, to allow for improved performance and reliability in the functioning device.
While computer simulations are useful to characterize the chip, it is desirable to measure the actual values of the internal current for a better understanding of the chip. However, there is currently no way to measure the current flow in semiconductor interconnection lines ("interconnect" lines) without cutting the lines or altering the lines in some way. While Hall probes can be used to measure currents on printed circuit boards, these devices typically have a spatial resolution on the order of several thousandths of an inch (mils), which is completely inadequate for semiconductor devices. Magneto transistors and magneto diodes have greater sensitivity than Hall probes and are smaller than Hall probes, but are still too large to provide the necessary spatial resolution for probing interconnect lines. Another approach is to use super conductive quantum interference devices (squids) which comprise two Josephson Junctions arranged back-to-back. One problem with this method is that it requires extremely low temperatures, so that the system is difficult to implement and costly. A further method of measuring current is E-beam probing. However, it is very difficult to separate the voltage contrast from the magnetic contrast, so that this method may not be sufficiently sensitive for measuring currents in interconnect lines. Additionally, since this must be carried out in a vacuum, this method requires the construction of a prober interface which can be placed under vacuum. Another method which has been proposed is use of magnetic force microscopy. In this method, a magnetized probe tip attached to a cantilever is scanned across an interconnect line. The deflection of the cantilever is used to determine the magnetic field, which is used to calculate current. One drawback of this method is that the frequency of the current must be approximately the same as the natural resonance frequency of the cantilever. A further drawback of this method is the relatively low sensitivity for dc currents.
What is needed is a method and apparatus for measuring current flow in an interconnection line of a semiconductor device. The method and apparatus should provide for measurement of current flow without alteration of the interconnection line. Further, the method and apparatus itself should not effect the current flow through the interconnection line, so that the measurement is an accurate reflection of current flow in the functioning device. The method and apparatus should allow for measurement of current flow without requiring that the device being tested, and/or any associated equipment be placed under vacuum or be placed in a very low temperature environment. The method and apparatus should have a spatial resolution- sufficient to measure current flow in an interconnection line typical of semiconductor devices, such as microprocessors. The method and apparatus should be insensitive to current flows through other closely spaced interconnection lines to the line of interest or should provide a method of determining the contribution of these closely spaced line to the measured current, to allow for calculation of the current through the line of interest alone. The method and apparatus should be sufficiently sensitive to provide accurate measurement of the ac and dc components of currents typical of semiconductor device interconnection lines.